Compression of motor unit recruitment threshold patterns is present in the subacute phase poststroke

Voluntary contraction anomalies of poststroke survivors progress from flaccid paralysis to recovery of upper extremity motor function in the subacute phase. However, muscle weakness often persists, and it is unclear what changes or aberrations persist in neuromuscular function, particularly in motor unit behavior. Our objective was to characterize motor unit discharge behavior in patients with hemiplegic stroke in the subacute phase. We tested seven patients with subacute stroke at two timepoints (timepoints 1 and 2), a minimum of 2 wk apart during the subacute phase. We used wireless surface electromyography to detect motor unit activities on both sides of our tested participants. Participants carried out two types of target force-tracking tasks with isometric elbow flexion. We performed a two-way ANOVA between the timepoint and test side. The recruitment threshold force (RTF) of the ramp task exhibited a significant interaction between the timepoint and test side (P < 0.00). The post hoc test showed the RTF of the affected side was not significantly lower than the contralateral side (P = 0.99) at timepoint 1. On the contrary, the affected side at timepoint 2 was significantly lower than the contralateral side (P < 0.00). The low recruitment threshold on the affected side may be more exacerbated than the contralateral side chronologically during the subacute phase of stroke. Our results suggest that the assessment of motor units in the subacute phase of stroke can contribute to the early detection of abnormal neuromuscular activity and, thereby, the establishment of effective rehabilitation.NEW & NOTEWORTHY This study clarified altered chronological motor unit recruitment patterns in the subacute stroke. We revealed that the neuromuscular physiological abnormalities on the affected side may persist from the subacute period to the chronic stage. To maximize recovery of motor function in patients with prolonged stroke symptoms, it is necessary to detect neuromuscular dysfunction in the subacute phase and establish early prevention. This study provided fundamental knowledge on the preventive rehabilitation of persistent paresis during the subacute phase.